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APPENDIX 2G
GUIDELINES AND RECOMMENDATIONS
Chapter 2
Helical Foundation Systems
friction angle. Laboratory direct shear tests or triaxial tests provide even more accurate estimates
of soil strength which may be warranted for large projects. The additional cost of performing these
tests could be offset by a more economical pier design that would not have been considered
using SPT results alone.
4. SPT N-values may be inconsistent for fine-grained, cohesive soils and may not accurately reflect
the soil shear strength. Tests may also be conducted on intact cohesive soil samples with pocket
penetrometers. These results can vary widely between technicians depending upon the accuracy
of the instrument and how closely the test procedure is followed. Laboratory testing of cohesive
samples collected using undisturbed sampling methods, such as Shelby tube sampling (ASTM
D1587), provides more reliable results. The more common methods for laboratory testing of
undisturbed samples of cohesive soils are the unconfined compression test, the triaxial shear
test and the direct shear test. Undrained shear strengths in excess of 4,000 pounds per square
foot (psf) are typically required to provide end-bearing resistance for push piers.
5. The presence of very loose granular soil and very soft to soft cohesive soil should be identified.
Column buckling may be a concern when very loose or soft soil is present in layer thicknesses
of more than just a few feet, especially for a deep foundation element that derives its capacity
primarily from end-bearing. Column buckling should be considered when SPT N-values are less
than 4 blows/foot. When SPT sampling indicates weight-of-hammer (WOH) or weight-of-rod
(WOR) values, additional laboratory testing for soil strength is recommended to document the
shear strength in the WOH/WOR zone. Cone penetrometer tests (CPT) completed in accordance
with ASTM D3441 may also be considered to measure in situ soil strengths. The CPT is widely
used in lieu of, or as a supplement to, the SPT. The CPT is particularly suited for soft clays, soft
silts and fine to medium sand deposits.
6. The presence of rubble fill, construction debris, or fill soils containing cobbles or boulders should
be identified. Large, hard fractions within fill soils would likely stop advancement of the push pier.
Pre-drilling may be required to allow the push pier to penetrate these soils and reach a suitable
bearing stratum.
7. When project characteristics or site conditions warrant, the Owner and his/her team of Design
Professionals may elect to determine the corrosive characteristics of the soils. Geotechnical or
environmental consultants may classify soils as corrosive based on visual review of soil samples,
from soil survey maps of the area, or from the results of additional testing. At a minimum, pH and
resistivity testing are required for a corrosion analysis. Multi-directional field resistivity testing is
preferred over laboratory resistivity testing. For a more complete corrosion analysis, chemical
analyses may be completed to determine specific concentration levels. Corrosive characteristics
of the soil should be determined from the ground surface to the bearing elevation of the deep
foundation, if practical.
Helical Piles/Anchors
1. Helical piles and anchors are advanced into the ground by the application of torque and crowd.
Crowd (or force) is applied longitudinally with the shaft to initiate penetration into the soil with
the helix bearing plates. Less crowd is typically needed after the helix plates advance a few feet
below the surface. Additional crowd may be needed in order to maintain the proper penetration
rate of about 3 inches per revolution (for a helix plate with a 3-inch pitch) through stiffer/denser
soil layers. Helical piles and anchors are best suited for medium dense sands and stiff to very
